The present invention is directed to a method and apparatus for indirectly measuring the solid-liquid interface equilibrium temperature by the dynamic excitation of the heat flux flowing through a "coldfinger" arrangement followed by a statistical estimation of the interface temperature, solid layer heat conductivity and thickness ratio, bulk liquid temperature and thermal heat transfer coefficient.
The aluminum industry would find it very valuable to have an accurate technique for determining, in real time, the cryolite liquid temperature, concentration of alumina, and bath temperature during the operation of an aluminum reduction cell. Such measurements could be used to control the operation of the aluminum reduction cell so as to optimize its operation and reduce the amount of energy needed.
Direct measurements of the various temperatures in the aluminum reduction cell are possible. However, the high temperatures involved and corrosive properties of the molten cryolite liquid require the use of expensive temperature measuring devices (e.g.--specially cladded thermocouples) having short lifetimes.
In view of the fact that other operational parameters in an aluminum reduction cell have been successfully estimated using simple mathematical models and on-line parameter estimation (e.g.--the ohmic resistance of the cryolite bath and the effective interelectrode gap), the present invention has been developed specifically for the accurate on-line estimation of the liquid temperature, alumina concentration, and average temperature of the cryolite bath. The invention is, of course, also applicable to other analogous molten bath situations, or solid-liquid equilibrium phenomena.
The alumina concentration of the cryolite bath can be estimated using a cryolite phase diagram with constant composition of all additives except alumina and an accurate estimate of the cryolite solid-liquid interface temperature. The basis of the interface temperature estimation is the observation of the dynamic heat transfer from the cryolite bath through a layer of frozen cryolite. This heat transfer is excited through the use of a "coldfinger" arrangement.
The coldfinger thermometer pocket excites the heat transfer from the bath through a layer of frozen cryolite and a tube wall to a gas flowing through the coldfinger. Excitation is accomplished by a periodic fluctuation of the inlet gas temperature and/or flowrate.
The inlet and exit temperatures of the gas are monitored using standard thermocouples.
Since the standard thermocouples are only subjected to the gas flowing through the coldfinger, expensive corrosion resistant devices with short lives are not needed.